The present invention relates to an apparatus and method for depositing durable coatings, and, more particularly, to the deposition of refractory carbide coatings onto the surface of a substrate by using lasers to heat the substrate and increase the reactivity of metal halide deposition gases.
The use of durable materials such as refractory carbide coatings to protect and/or seal various substrates from the external environment is well established and includes such applications as nuclear fuel encapsulation and radioactive waste containment. Nuclear fuels such as uranium rods, pellets, or particles are often encapsulated in refractory carbide coatings to protect the fuel interior from the high temperatures that are produced within a nuclear reactor and to prevent the release of radioactive fission products from within the fuel structure. In particular, durable carbide coatings are useful in protecting small diameter fuel particles commonly used in high temperature gas reactors such as particle bed reactors where the normal operating temperatures can exceed 2000 degrees Kelvin. Similarly, containing or storing long term radioactive waste within a durable refractory carbide coating presents an attractive alternative to storing such waste in cement or concrete structures which may prove to be unstable in the long run.
Typically, refractory carbide coatings are applied to a graphite or a carbon composite substrate by using conventional chemical vapor deposition (CVD) or chemical vapor reaction (CVR) methods. The latter process, CVR, has proven to be especially effective in producing hard and dense carbide coatings. In this process a metal halide vapor reacts with the carbon in the substrate to produce the carbide. For instance, the reaction for depositing a coating such as tantalum carbide is: EQU TaCl.sub.5 +C.sub.(substrate) .fwdarw.TaC+5/2Cl.sub.2
The reaction normally produces a carbide coating with a thickness of about 10 to 100 micrometers.
The process, however, requires extremely high temperatures, and depending on the carbide to be produced, the typical reaction such as shown above occurs optimally at a temperature range of about 1700 K. to 2600 K. The high temperatures present a problem in both nuclear fuel encapsulation and radioactive waste containment, since, in conventional CVD and CVR methods, the entire substrate is normally subjected to such high temperatures. With nuclear fuel, the high temperature may damage or weaken the structural integrity of the uranium fuel core as well as the buffer layer of pyrolytically deposited carbon and cause a deformation of the fuel structure with possible leakage of uranium or other fissile products from the core. In containing radioactive wastes, high temperatures create even a greater problem, since the waste, being typically in the form of an ion-exchange resin, is even more unstable at high temperatures.
Further, as the coating is produced at high temperatures, the grain size of the carbide material will naturally be large. Since grain size is related to porosity, large grain carbide coatings may more readily allow radioactive gases to diffuse out of the substrate or, conversely, allow external gases to diffuse into the substrate.
Finally, conventional CVD or CVR methods are not generally appropriate for performing spot repairs or depositing coatings of a predetermined pattern or shape. In the conventional methods, since the entire substrate is heated and exposed to the halide or other vapors, material deposition will take place in a random fashion wherever the vapors reacts with the substrate.
In view of the foregoing, the general object of this invention is to provide an apparatus and method for depositing a coating on a substrate without subjecting the substrate to the high temperatures associated with conventional CVD and CVR methods of deposition.
Another object of this invention is to provide an apparatus and method for depositing refractory carbide coatings which are dense and resistant to high temperatures.
Yet another object of this invention is to provide an apparatus and method for depositing coatings which can perform "spot repairs" on coatings and deposit precise coatings of predetermined shape or pattern.
Additional objects, advantages and novel features of the invention will become apparent to those skilled in the art upon examination of the following and by practice of the invention.